1. Field of the Invention
The present invention relates to a micro-oscillation element, such as a micro-mirror element having a movable section capable of rotational displacement, and also relates to a drive method for such a micro-oscillation element.
2. Description of the Related Art
In recent years, it has been sought to apply the use of elements having extremely small structures formed by micro-machining technology, in various technological fields. For example, in the field of optical communications technology, attention has focused on very small micro-mirror elements which have a light reflecting function.
In optical communications, an optical signal is transmitted by using an optical fiber as a medium, and furthermore, in general, a so-called optical switching device is used in order to switch the transmission path of the optical signal from one fiber to another fiber. Characteristics required in an optical switching device in order to achieve good optical communications include high capacity, high speed and high reliability, in the switching operation, and the like. From this point of view, expectations have been growing with regard to optical switching devices which incorporate micro-mirror elements fabricated by micro-machining technology. This is because the use of a micro-mirror element makes it possible to carrying out switching processes on the optical signal itself, without having to convert the optical signal to an electrical signal, between the optical transmission path on the input side of the optical switching device and the optical transmission path on the output side thereof, and this means that it is suitable for obtaining the characteristics described above.
A micro-mirror element is provided with a mirror surface for reflecting light, and it is capable of changing the direction in which the light is reflected by oscillation of the mirror surface. Electrostatic drive-type micro-mirror elements which use electrostatic force in order to cause the mirror surface to oscillate are used in many devices. Electrostatic drive-type micro-mirror elements can be divided broadly into two types: micro-mirror elements manufactured by so-called surface micro-machining technology, and micro-mirror elements manufactured by so-called bulk micro-machining technology.
In the case of surface micro-machining technology, a thin layer of material corresponding to a respective constituent area is formed on a substrate and processed into a prescribed pattern, and such patterns are layered in a sequential fashion, whereby respective areas constituting an element, such as a support, mirror surface and electrode section, and the like, are formed, and a sacrificial layer which is subsequently removed is also formed. On the other hand, in the case of bulk micro-machining technology, a support and mirror section, and the like, are formed in a prescribed shape by etching the actual material of the substrate, mirror surfaces and electrodes being formed as thin layers thereon according to requirements. Bulk micro-machining technology is described, for example, in Japanese Patent Laid-Open No. (Hei)10-190007, Japanese Patent Laid-Open No. (Hei)10-270714 and Japanese Patent Laid-Open No. 2000-31502.
One technical feature required in a micro-mirror element is that the mirror surface which performs light reflection has a high degree of flatness. However, in the case of surface micro-machining technology, since the mirror surface ultimately formed is thin, the mirror surface is liable to curve, and consequently, it is difficult to achieve a high degree of flatness in a mirror surface having a large surface area. On the other hand, in the case of bulk micro-machining technology, a mirror section is constituted by cutting into the actual material substrate, which is relatively thick, by means of an etching process, and since a mirror surface is provided on this mirror section, it is possible to ensure rigidity, even if the mirror surface has a large surface area. Consequently, it is possible to form a mirror surface having a sufficiently high degree of optical flatness.
FIG. 43 and FIG. 44 illustrate a conventional electrostatically driven micro-mirror element X8 fabricated by means of bulk micro-machining technology. FIG. 43 is an exploded oblique view of a micro-mirror element X8, and FIG. 44 is a cross-sectional view along line XXXXIV—XXXXIV in FIG. 43 of the micro-mirror element X8 in an assembled state.
The micro-mirror element X8 has a structure in which a mirror substrate 80 and a base substrate 86 are layered on each other. The mirror substrate 80 is constituted by a mirror section 81, a frame 82, and a pair of torsion bars 83 linking same together. By performing etching from one side only on a prescribed material substrate, such as a silicon substrate having electrical conductivity, it is possible to form the outline shape of the mirror section 81, frame 82 and torsion bars 83, on the mirror substrate 80. A mirror surface 84 is provided on the surface of the mirror section 81. A pair of electrodes 85a, 85b are provided on the rear face of the mirror section 81. The pair of torsion bars 83 define an axis A8 for the rotational operation of the mirror section 81, as described hereinafter. An electrode 87a opposing the electrode 85a of the mirror section 81, and an electrode 87b opposing the electrode 85b are provided on the base substrate 86.
In the micro-mirror element X8, when an electric potential is applied to the frame 82 of the mirror section 80, the electric potential is transmitted to the electrode 85a and the electrode 85b, by means of the pair of torsion bars 83 and the mirror section 81, which are formed in an integral fashion from the same conductive material as the frame 82. Consequently, by applying a prescribed electric potential to the frame 82, it is possible to charge the electrodes 85a, 85b, positively, for example. In this state, if the electrode 87a of the base substrate 86 is charged with a negative charge, then an electrostatic attraction is generated between the electrode 85a and the electrode 87a, and hence the mirror section 81 rotates in the direction of the arrow M8, as indicated in FIG. 44, whilst twisting the pair of torsion bars 83. The mirror section 81 is able to swing until it reaches an angle at which the force of attraction between the electrodes balances with the sum of the twisting resistances of the respective torsion bars 83. Alternatively, if a negative charge is applied to the electrode 87b whilst a positive charge is being applied to the electrodes 85a, 85b of the mirror section 81, then an electrostatic attraction is generated between the electrode 85b and the electrode 87b, and hence the mirror section 81 will rotate in the opposite direction to the arrow M8. By driving the mirror section 81 to swing as described above, it is possible to switch the direction of reflection of the light reflected by the mirror surface 84.
In the micro-mirror element X8, in order to attain a large angle in the rotational displacement of the mirror section 81, it is necessary to ensure a sufficient gap between the mirror substrate 80 and the base substrate 86, in order to avoid mechanical contact between the mirror substrate 80 and base substrate 86. However, since the electrostatic force generated between the electrodes 85a and 87a, or between the electrodes 85b and 87b, tends to decline as the distance between the electrodes increases, the drive voltage that is to be applied between the respective electrodes must be increased to a corresponding degree, in order that the mirror section 81 can be driven suitably, whilst guaranteeing a sufficient gap between the mirror substrate 80 and the base substrate 86. In many cases, increasing the drive voltage is undesirable, in terms of the composition of the element, or from the viewpoint of reducing power consumption.
FIG. 45 is a partially abbreviated oblique diagram of a further conventional micro-mirror element X9, which is fabricated by means of bulk micro-machining technology. The micro-mirror element X9 has a mirror section 91 provided with a mirror surface 94 on the upper surface thereof, a frame 92 (partially omitted), and a pair of torsion bars 93 for linking same together. Comb tooth-shaped electrodes 91a, 91b are formed at the two respective end portions of the mirror section 91. A pair of comb tooth-shaped electrodes 92a, 92b are formed in the frame 92, extending in an inward direction in positions corresponding to the comb tooth-shaped electrodes 91a, 91b. A pair of torsion bars 93 define an axis A9 for the rotational operation of the mirror section 91 with respect to the frame 92.
In a micro-mirror element X9 having a composition of this kind, the set of comb tooth-shaped electrodes provided in adjacent positions in order to generate an electrostatic force, for example, the comb tooth-shaped electrode 91a and the comb tooth-shaped electrode 92a, are oriented in a two-tier fashion as illustrated in FIG. 46A, when no voltage is applied to same. On the other hand, when a prescribed voltage is applied, as illustrated in FIG. 46B, the comb tooth-shaped electrode 91a is drawn inside the comb tooth-shaped electrode 92a, thereby causing the mirror section 91 to rotate. More specifically, for example, if the comb tooth-shaped electrode 91a is charged with a positive charge, and the comb tooth-shaped electrode 92a is charged with a negative charge, then the mirror section 91 will rotate about the axis A9, whilst twisting the pair of torsion bars 93. By driving the mirror section 91 to swing in this fashion, it is possible to switch the direction of reflection of the light reflected by the mirror surface 94 provided on the mirror section 91. It is known that the drive voltage required in order to drive a pair of comb tooth-shaped electrodes of this kind tends to be lower than the drive voltage required to achieve driving of a pair of planar electrodes, as in the micro-mirror element X8 described above.
FIG. 47 shows a method for manufacturing a micro-mirror element X9. In FIG. 47, the process of forming a portion of the mirror section 91 shown in FIG. 45, and the frame 92, torsion bars 93 and a portion of the set of comb tooth-shaped electrodes 91a, 92a, is illustrated by the changes in a particular cross-section. The cross-section represents a continuous cross-section which is modeled on a plurality of cross-sections that are contained within the section in which a single micro-switching element is formed on a material substrate (wafer) that is subjected to various processes.
In the method of manufacturing a micro-mirror element X9, firstly, a wafer S9 is prepared as illustrated in FIG. 47A. The wafer S9 is a so-called SOI (Silicon on Insulator) wafer, which has a laminated structure comprising a silicon layer 901, a silicon layer 902 and an insulating layer 903 interposed between these layers. Next, by carrying out anisotropic etching via a prescribed mask on the silicon layer 901, as illustrated in FIG. 47B, the constituent parts that are to be formed in the silicon layer 901 (the mirror section 91, a portion of the frame 92, the torsion bars 93, and the comb tooth-shaped electrode 91a) are formed. Next, by carrying out anisotropic etching via a prescribed mask on the silicon layer 902, as illustrated in FIG. 47C, the constituent parts that are to be formed in the silicon layer 902 (a portion of the frame 92, and the comb tooth-shaped electrode 92a) are formed. Subsequently, as illustrated in FIG. 47D, the exposed portion on the silicon layer 903 is removed by carrying out isotropic etching on the insulating layer 903. In this way, the mirror section 91, frame 92, torsion bars 93, and the set of comb tooth-shaped electrodes 91a, 92a, are formed. The other set of comb tooth-shaped electrodes 91b, 92b are formed in a similar manner to the comb tooth-shaped electrodes 91a, 91b. 
In the micro-mirror element X9, since the comb tooth-shaped electrodes 91a, 91b are displaced in accordance with the rotational operation of the mirror section 91, the comb tooth-shaped electrodes 91a, 91b must have sufficient thickness corresponding to the prescribed angle of inclination of the mirror section 91. Therefore, in order to achieve a large angle in the rotational displacement of the mirror section 91 of the micro-mirror element X9, it is necessary to design the comb tooth-shaped electrodes 91a, 91b to be long in the direction of rotational operation, thus ensuring a sufficient length for the stroke of the drive electrodes (the stroke being the range of relative movement of the electrode pair in the direction of the rotational operation which is tolerable whilst still being able to generate a suitable driving force). In order to ensure a long stroke, in the aforementioned method of manufacture, it is necessary to carry out processing on a material substrate S9 having silicon layers 901, 902 of a thickness which corresponds to the required stroke length. However, it tends to be difficult to form comb tooth-shaped electrodes 91a, 91b wherein each electrode tooth has a relatively small width, to a high degree of accuracy, by carrying out etching processes, or the like, on relatively thick silicon layers 901, 902.
In addition, in the micro-mirror element X9, since the mirror section 91 is formed to the same thickness as the comb tooth-shaped electrodes 91a, 91b, then the formation of comb tooth-shaped electrodes 91a, 91b which are long in the direction of rotational operation inevitably leads to the formation of a thick mirror section 91. The thicker the mirror section 91, the greater the mass of the mirror section 91 and hence, the greater the inertia thereof. Consequently, cases have occurred in which it is not possible to achieve the desired speed in driving the rotational operation of the mirror section 91.
In this way, in a conventional micro-mirror element X9, it has been problematic to achieve rotational operation of the mirror section 91 involving a large amount of rotational displacement, at a high speed of operation.